The present invention relates to an X-ray source having a rotary anode, particularly to the structure of a member constituting a V-grooved target portion in the anode.
Among several applications of X-ray radiation, a lithography using soft X-rays having a wave length in the range from few to tens of angstroms is drawing a great attention in the semiconductor manufacturing industries. Such X-rays allow high precision transcription of fine semiconductor circuit patterns of micron or submicron on a substrate such as a silicon wafer, because of its less interference characteristic compared with the visible light used in the conventional photolithography.
An electron bombardment is usually employed for generating X-rays, wherein an anode or the target portion thereof, formed from an X-ray emissive material, is bombarded by a high energy electron beam. The X-ray sources using electron bombardment include a fixed or stationary anode type and a rotary anode type. In these types of X-ray sources, more than 99% of the energy of the incident electron beam is converted into heat and only the remainder energy is utilized to generate X-ray radiation. Therefore, to increase efficiencies in the conversion of electron beam energy into X-ray radiation and in the removal of heat dissipated at the anode or target portion is a crucial problem in the design of electron bombardment type X-ray sources.
A rotary anode X-ray source is designed to alleviate the heat dissipation problem. The apparent area of the target portion in a rotary anode is relatively increased and, therefore, the mean value of the electron beam power density on the target area can be kept low compared with that on a stationary anode. Thus, a rotary anode X-ray source can be operated under an input electron beam power as much as 100K-Watts, compared with the allowable input electron beam of about 10K-Watts in a stationary anode type source, thereby providing an X-ray emission of greater intensity.
To increase the conversion efficiency of the electron beam energy to an X-ray emission, a rotary anode having a V-grooved target portion was proposed. There are several disclosures of this type of X-ray source, including the U.S. Pat. Nos. 4,336,476 published June 22, 1982 and 4,405,876 published Sept. 20, 1983, and Japanese patent applications Tokukaisho No. 59-205139 published Nov. 20, 1984, Tokukaisho No. 59-221950 published Dec. 13, 1984 and Tokukaisho 60-254540 published Dec. 16, 1985. Referring, for example, to the Tokukaisho No. 60-254540, the inventor of which application is the co-inventor of the present invention, it is described that the X-ray conversion efficiency of a rotary anode is increased by providing therefor a V-grooved target portion, because the back-scattered electrons are almost absorbed during their multiple collisions with the surface of the V-grooved target. Further, the uniformity of the X-ray field intensity distribution can be improved by the use of a V-grooved target.
FIG. 1 shows an X-ray tube disclosed by the above U.S. Pat. No. 4,336,476, wherein an anode target disc 11 rotated by a skirt-type rotor 12 is provided with a focal track groove 13 disposed in the peripheral rim surface 14 thereof. FIG. 2 shows a part of liquid cooled anode X-ray tube disclosed by the above U.S. Pat. No. 4,405,876, wherein a V-groove 21 is provided on the periphery of a rotating anode 22. The rotating anode 22 is cooled by liquid flowing through a space between the anode 22 and a stationary septum 23. FIG. 3 shows a rotating anode X-ray tube disclosed by the above Tokukaisho No. 59-221950, wherein a target 31 rotated by a rotor 32 is provided with a V-groove 33. FIG. 4 shows a rotating anode for a high power X-ray source disclosed by the above Tokukaisho No. 59-205139, wherein a V-groove formed on the periphery of a rotating circular anode 41 is provided with a backwardly extending hollow portion 42 for eliminating the high power density of incident electron beam at the apex of the V-groove. FIG. 5 shows a rotary anode disclosed by the above Tokukaisho 60-254540, wherein a V-groove 51 provided on the periphery of a rotary anode 52 has a cross-section in which the direction of the normal line to the surface of the V-groove is not constant with respect to the direction of the incident electron beam 53 but varies from zero at the periphery of the V-groove 51 to approximately 90.degree. at the apex of the V-groove 51. A similar variable taper V-groove 62 is provided on a flat surface of a rotary anode 61, perpendicular to the electron beam 63 incident thereon, as shown in FIG. 6.
In any one of the above disclosures, the V-groove constituting an electron beam track (target portion) is formed by engraving a cylindrical or flat side surface of a rotary anode. However, such rotary anode or target portion has disadvantages as summarized below.
(a) Low mechanical strength of a rotary anode
(b) Difficulty in the machining of an anode having a V-groove formed therein.
(c) High input power density at the apex of the V-groove
(d) Poor adhesion of an X-ray emissive material layer formed on the surface of the V-groove.
These disadvantages will be discussed briefly in the following.
Firstly, a rotary anode is generally formed from such a material having a high mechanical strength and thermal conductivity as copper (Cu) or copper-based alloy, Cu-Cr, for example. However, when the rotary anode having a V-groove formed therein is rotated at a speed of few to ten thousands rpm, a stress is concentrated at the apex of the V-groove. As a result, if a flaw exists there, it grows to extend into the anode member in the radial direction, and finally results in causing the breakage of the rotary anode member. In addition, it is difficult to provide a sharp apex for a V-groove when the apex is formed by engraving an anode member. The apex of a V-groove generally has a surface portion formed substantially perpendicular to the incident electron beam. Such surface portion is inevitably burdened with an excessive input power density (power per unit area). Hence, temperature at the apex is raised and a heat stress is generated. Thus, the above-mentioned breakage of the rotary anode due to the growth of flaw is accelerated.
Secondly, a layer of an X-ray emissive material, aluminum (Al), silicon (Si) or palladium (Pd), for example, is generally formed on the surface of the V-groove, in order to provide a desired characteristic X-ray emission. However, if the X-ray emissive material layer of a thickness of one micron or more is formed on the surface of the V-groove uniformly by using an ion plating technology, it is difficult to assure a high adhesion strength between the X-ray emissive material layer and the V-groove surface, because, the adhesion strength of a layer deposited on a surface by an ion plating is maximum when ions impinge perpendicularly to the surface and decreases as the incidence angle of the ions becomes smaller. In the ion plating of the V-groove surface, ions inevitably impinge on the V-groove surface obliquely, relatively deviated from the the perpendicular condition. Thus, poor adhesion is established between the V-groove surface and the X-ray emissive material layer and the layer can not remain on the surface under the high speed rotation of the anode and the application of thermal stress caused by the electron bombardment applied thereto.
Thirdly, a rotary anode for an X-ray source is sometimes furnished with a cooling means therefor, as shown in one of the above cited disclosures. For this cooling, the anode assembly inevitably has a complicated structure usually composed of a member for constituting a target portion and another member for circulating fluid coolant for the target member having a V groove. Accordingly, machining of these members involves a great deal of difficulties.